At present, airbags are indispensable for ensuring the safety of drivers and passengers in automobiles, and the percentage of airbag installation in automobiles is increasing.
The requirement for improved reliability of airbags as safety devices is even more increasing, and the other requirements for compactness and cost reduction of airbag devices are also further more increasing. Accordingly, more improvements are required for base fabrics for airbags and for the process of producing yarns and base fabrics for airbags so as to satisfy the above mentioned requirements.
Techniques have heretofore been disclosed for realizing air bags that are compactly foldable to save the necessary housing space, not interfering with good mechanical properties of base fabrics for airbags. Above all, a technique of using a fabric of modified cross-section yarns for base fabrics for airbags is specifically noticed since it satisfies the capabilities such as safety and containability necessary to next-generation airbags.
The prior-art techniques of using modified cross-section yarns are, for example, in JP-A 4-193647, 4-201650, 7-252740, 8-60425, 2002-129444, and U.S. Pat. No. 6,037,047.
JP-A 4-193647 and 4-201650 disclose a base fabric for airbags which is lightweight, flexible and containable and has good mechanical properties, for which are used polyamide multifilaments composed of a plurality of modified cross-section filaments each having a filament fineness of from 1.0 to 12 deniers and having a degree of cross-section modification of from 1.5 to 7.0.
However, the technique disclosed is to utilize the entanglements and the loops formed in yarns for the surface characteristics of fabrics to thereby increase the surface area of the fabric and increase the efficiency of resin adhesion to the fabric, or that is, to increase the adhesiveness of resin to the fabric. In addition, the technique is characterized by increasing the bulkiness of fabrics to thereby increase the thickness of the fabrics that are still flexible. In other words, the technique has no intension of reducing the amount of resin to be adhered to fabrics and reducing the thickness of the fabrics.
JP-A 7-252740 discloses a base fabric for airbags, which is formed of multifilament yarns having a degree of filament cross-section flatness of at least 1.5, a total fineness of from 180 deniers to 450 deniers, a filament fineness of from 0.1 deniers to 7.0 deniers, a tenacity of at least 7.5 g/d, and an elongation of at least 12.0%.
It says that, according to the technique disclosed therein, a base fabric for airbags that is lightweight and containable and has low air permeability could be surely formed of yarns having a flattened filament cross-section and having a specific total fineness and specific filament fineness. However, the base fabric is not coated, or that is, the technique is for attaining reduced air permeability of non-coated base fabrics, and is not for attaining improved compactness and containability of coated base fabrics that are formed by coating base fabrics with resin.
JP-A 8-60425 discloses fibers for airbags, which are so designed that, in the filament cross-section, there exist from 1 to 3 nearly semi-circular projections symmetrically given in the major-axis direction of the flattened base of the cross-section, the ratio of the major axis to the minor axis of the flattened cross-section filament falls between 4/1 and 2/1, the filament fineness is from 2 to 10 deniers, and the tenacity is at least 7 g/d. However, the technique disclosed is for improving the producibility of the fibers by forming the projections in the filament cross-section thereof, while, on the other hand, the projections interfere with the packing capability of the filaments, and, as a result, the fabric formed of the fibers is unsatisfactory in point of reducing the thickness thereof. In addition, the reference is silent on resin coating, and discloses the technique that relates to non-coated base fabrics.
JP-A 2002-129444 discloses a technique for improving the containability and the air-imperviousness of non-coated base fabrics for airbags. However, this is silent on a technique of making the best use of the characteristics of flattened yarns as much as possible, or that is, this gives no description of indicating a technique for aligning the constitutive fibers in a base fabric in such a manner that the major axis of the warp and the weft to constitute the base fabric is highly oriented in the horizontal direction of the base fabric in the cross-section thereof.
U.S. Pat. No. 6,037,047 discloses the effectiveness of diamond-shaped or S-shaped flattened cross-section yarns for polyester fiber fabrics having good covering capability and flexibility, saying that the yarns of the type are useful for fabrics for airbags. The polyester fiber fabrics formed of the diamond-shaped or S-shaped flattened cross-section yarns described in the patent specification surely have good covering capability and are highly flexible although they are formed of polyester fibers, but when compared with fabrics of polyamide fibers, it could not be said that the polyester fiber fabrics are specifically effective. In addition, the patent specification gives no description of coated base fabrics for airbags, and it may be said that this discloses a technique of non-coated base fabrics for airbags.
On the other hand, recently, non-coated base fabrics with no resin coating have become widely popularized with the intensive increase in the requirement for compactness, containability and cost reduction of airbags. Non-coated airbags are surely advantageous over coated airbags in point of the compactness, containability and cost reduction, but are disadvantageous than the latter in point of the heat resistance and the gas imperviousness, and, in addition, the safety of the former would be problematic as compared with that of the latter. In particular, the output power of inflators is being increased these days for further increased airbag safety. With that, therefore, base fabrics for airbags are required to have more increased heat resistance and air imperviousness, and at their most extreme, base fabrics are required to have a air permeability of 0 (zero). Given that situation, coated base fabrics with resin coating formed on the surface of base fabrics are still useful for airbags.
It could therefore be advantageous to provide a base fabric for airbags that has well-balanced properties of flame resistance, complete air-imperviousness and compactness that could not be attained by any conventional non-coated or coated base fabrics for airbags and to provide a coated base fabric for airbags that satisfy both good safety and good containability, or that is, for those which are thin and have good mechanical properties and good heat resistance and, when they have expanded, release substantially no gas except through the vent hole thereof, and which have good flexibility, compactness and containability.